Thermoplastic resins are commonly provided for commercial use in the form of pellets. Various pelletizing methods have been employed after the plastic mass formed in a polymerization reaction has been forced through a die under sufficient pressure. In what is known as the cut strand method a die with a large number of small holes is used, the hole being of various geometrical configurations including round, square and triangular shapes. After the plastic mass has been forced through the die, the strands formed are cooled and then chopped into short pieces by a rotary blade cutter. The pellets obtained are in the form of short cylinders or prisms. However, it is difficult in high rates of production to satisfactorily handle the large number of strands which are involved.
In the die face cut method, instead of drawing a strand from the die, however, the hot extrudate is cut off at the die face by a rapidly moving blade. This is commonly carried out underwater and the procedure is, therefore, known as underwater pelletizing. It is also possible to realize the same results by extruding the plastic mass into air and providing for the cutter to throw the pellets into water as soon as they are cut in order to prevent the hot pellets from sticking together. This is called hot face pelletizing. This method produces pellets which are generally a distorted spherical shape and is a frequently employed method of pelletizing polyolefin plastics. It is an especially desirable method for use when the formed plastic material is to be transferred by an air conveyor since the spherical shape generally does not result in the production of as large an amount of fines during transfer.
However, the pellets heretofore available by the practice of previously known procedures suffered from the disadvantages of a non-uniform pellet geometry and by the presence of imperfections in the form of adhering solidified chunks or strands. The procedure also resulted in plugged holes in the die. These disadvantages resulted in the generation of high proportion of scrap during the production of resin pellets. With certain difficultly processable materials such as some types of polypropylene the amount of scrap generated could amount to from 5% to 20% of total production.
Various means have been employed in the prior art to achieve the production of more uniformly shaped pellets and to prevent the development of imperfections in the pellets. An illustrative prior patent is Guill U.S. Pat. No. 3,029,466 of 1962. Neither this patent or any prior art known to applicants provide the economic and efficient means for the provision of uniformly shaped pellets which comprises the subject matter of this invention.